Technological advancements of the last several years have enabled satellite based systems to offer voice and data services to mobile terminals on a global basis. These systems can also provide voice and data services to fixed installation terminals, thereby enabling basic telephony services in areas lacking a terrestrial telecommunications infrastructure. Primary objectives of these personal communication satellite services (PCSS) are to offer the services at low prices and to provide a high level of service quality.
In recent years satellite based systems have been proposed which offer direct communication between mobile or fixed terminals and satellites arranged at low and medium altitudes. The communications links include traffic channels over which voice information or data are transmitted. Proposed satellite based telecommunications systems utilize earth stations to interconnect through gateways with conventional terrestrial phone networks. The earth stations may also route communications between satellites and terminals. The earth stations may further provide control over signaling, transmission timing and transmission frequency of the terminals as necessary to establish and maintain calls directed to or initiated by terminals.
Examples of recently proposed satellite based systems include the Globalstar.TM. system proposed by Globalstar.TM. Telecommunications Limited, the Iridium system proposed by Motorola Inc., and the Odyssey system proposed by TRW and Teleglobe.
The earth stations, satellites and mobile terminals communicate via a predefined waveform format. The waveform format supports forward communications links from earth stations through satellites to terminals. The waveform format also supports return communications links from the terminal through the satellite to the earth station. The design of the communications waveform format for a system plays a significant role in meeting the system's objectives such as enhancing bandwidth efficiency, enhancing satellite power usage efficiency, providing rapid terminal acquisition, providing robust communications links and maintaining user privacy. Further objectives include maximizing the number of simultaneous terminals that a system may support while minimizing the capital cost of the system. The number of terminals supported by a single satellite depends in part on the available bandwidth for communications between the satellite and terminals and between the satellite and earth stations. The number of terminals also depends upon the power required by each terminal, the satellites' RF transmission capability, physical environment factors (e.g., necessary link margins), regulatory constraints (e.g., terminal radiated power constraints, satellite power flux density constraints, out-of-band emissions constraints, etc.) and the like.
Communications waveform formats have been proposed in the past, such as the Telecommunication Industry Association/Electronics Industries Association Interim Standard 95 (IS-95) proposed by QUALCOMM, Inc., of San Diego, Calif., with some cooperative effort from AT&T, Motorola and others. IS-95 incorporates CDMA modulation techniques disclosed in U.S. Pat. No. 5,103,459. IS-95 describes a code division multiple access (CDMA) waveform format, in which multiple terminals communicate in a common bandwidth or subband. In this common subband, terminals are distinguished from one another by a code uniquely assigned to each terminal. The CDMA code may also be referred to as a codeword or "chip code". The chip code represents a pseudo-noise (PN) spreading code which "spreads" the signal over the available bandwidth and allows more terminals to communicate over the same frequency range. The chip code is combined or modulated with information bits which define a voice or data signal. The combined data stream of voice or data and the chip code is divided into frames and transmitted over a traffic channel. The chip code is transmitted at a rate (the chip rate) much faster than the information bit rate.
In the IS-95 waveform format, a single CDMA subband is 1.23 MHz wide and will support a theoretical maximum of 63 terminals or subscribers with unique CDMA codes. In practice, the transmissions to and from the terminals interfere with one another and unduly degrade the quality of each communications link if more than approximately 30 terminals share a subband for satellite application. In terrestrial application as few as 12 terminals may be able to share a subband. This type of interference is referred to as "multiple access interference". The IS-95 waveform format and CDMA generally are explained in more detail in chapter 13 of a book entitled "An Introduction to GSM", by Siegmund H. Reidl, Matthias K. Weber and Malcolm W. Oliphant, published by Artech House, Inc., of Norwood, Mass., 1995. Chapter 13 of the above-referenced book is expressly incorporated herein by reference.
However, CDMA systems thus far proposed have met with limited success. By way of example, the IS-95 waveform affords an asynchronous return link (i.e., from the terminal to the earth station) which unduly limits the number of terminals that may simultaneously communicate over a limited bandwidth.
In "asynchronous" CDMA terminals transmit communications to an earth station independent in time from one another. This results in far larger multiple access interference than may result with orthogonal CDMA.
Further, the IS-95 waveform uses a combination of "open loop" and "closed loop" methods for controlling signal power transmitted by the terminal in the return link. The terminal may adjust return link transmission power in part based on the power received on the forward link (i.e., open loop control). However, this open loop power control routine is inaccurate since power fluctuations of the signals on the forward and return links are not necessarily correlated to one another.
Moreover, the IS-95 waveform makes inefficient use of the available bandwidth by requiring every frame in the traffic channel to include "tail bits" to convert the convolutional code into a block code. These tail bits reduce the transmission rate for voice or data.
A need remains within the industry for an improved communications waveform for use in satellite based cellular telecommunications.